Protective masks are used in a wide variety of applications to protect the human's respiratory system from particles suspended in the air, from powders as well as from solid and liquid aerosols.
The masks generally fall into two categories, molded cup-shaped masks and fold-flat masks.
Molded cup-shaped masks are described, for example, in GB-A-1 569 812, GB-A-2 280 620, U.S. Pat. No. 4,536,440, U.S. Pat. No. 4,807,619, U.S. Pat. No. 4,850,347, U.S. Pat. No. 5,307,796, U.S. Pat. No. 5,374,458.
Fold-flat masks, which can be kept flat until needed, are described, for example, in WO 96/28217, in U.S. patent application Ser. No. 08/612,527, in U.S. Pat. No. 5,322,061, U.S. Pat. No. 5,020,533, U.S. Pat. No. 4,920,960 and U.S. Pat. No. 4,600,002.
The masks are formed from one or more layers of air-permeable materials, typically from an inner layer, a filtering layer and a cover layer.
The filtering layer is normally made from a non woven fabric, in particular from melt-blown microfibers, as disclosed, for example, in U.S. Pat. No. 5,706,804, U.S. Pat. No. 5,472,481, U.S. Pat. No. 5,411,576 and U.S. Pat. No. 4,419,993. The filter material is typically polypropylene.
The filtering material may also contain additives to enhance filtration performances such as, for example, the additives described in U.S. Pat. No. 5,025,052 and U.S. Pat. No. 5,099,026.
The material may also incorporate moisture and mist resistant agents (U.S. Pat. No. 4,874,399, U.S. Pat. No. 5,472,481, U.S. Pat. No. 5,411,576) or electric charge can be imparted to the material (U.S. Pat. No. 5,496,507, U.S. Pat. No. 4,592,815, U.S. Pat. No. 4,215,682).
The outer coverweb protects the filtering layer from abrasive forces. The coverweb is normally made from non woven fibrous materials, typically from polyolefins, polyesters or polyamides. Examples are described in U.S. Pat. No. 4,807,619 and U.S. Pat. No. 4,536,440.
The inner layer has shape-retaining function and is normally made from non woven fabric, typically from polyester.
When the air passes through the mask, the filtering layer removes the contaminants from the flow stream preventing the wearer from inhaling them. Analogously the exhaled air, passing through the mask, is purged from pathogenous agents and from contaminants preventing other persons from being exposed.
Some masks are equipped with an exhalation valve which opens, when the wearer exhales, in response to increased pressure, while closes, during inhaling, forcing the air to pass through the filtering medium.
Examples of masks equipped with valves can be found in U.S. Pat. No. 4,827,924, U.S. Pat. No. 347,298, U.S. Pat. No. 347,299, U.S. Pat. No. 5,509,436, U.S. Pat. No. 5,325,892, U.S. Pat. No. 4,537,189, U.S. Pat. No. 4,934,362, U.S. Pat. No. 5,505,197, US 2002023651.
In order to improve the seal between the mask and the face, the masks may also include additional features such as nose clips, as described in U.S. Pat. No. 5,558,089, and bands, as described in U.S. Pat. No. 4,802,473, U.S. Pat. No. 4,941,470 and U.S. Pat. No. 6,332,465.
Despite the several kinds of available masks, continues efforts are being made in finding new protective means having improved properties in comparison with the existing art.